Tomograph

ABSTRACT

An X-ray CT apparatus takes a short time for image processings in irradiating a cone beam X-ray to the whole jaw, including a dental arc, and to a local region, and to obtain a panoramic image showing conditions of a dental row. The apparatus includes, a rotative arm suspended at one end of a support column, and an X-ray generating device and a two-dimensional X-ray detecting device oppositely fixed to respective ends of the rotative arm. The rotative arm is supported via a dual rotation system. An object to be examined sits on a chair which is movable upward and downward, and an examining region is adjusted to the height of the imaging center of the imaging apparatus. The object&#39;s head is fixed by a fine-adjustable head holder. Imaging is performed so that the distance between rotation centers of the two rotation systems substantially coincides with the object&#39;s dental arc.

TECHNICAL FIELD

The present invention relates to an X-ray CT apparatus for obtaining atomographic image and the like by irradiating an X-ray to a portion ofan object to be examined and processing a projection image of theregion; and, more particularly, the invention relates to an X-ray CTapparatus which can obtain an arbitrary CT image and a panoramic imageof a region by irradiating a cone beam X-ray, which is suitable forimaging in dental examination.

BACKGROUND OF THE INVENTION

In current dental examination, general imaging in which a film is heldat the back of teeth to perform X-ray imaging, panoramic imaging inwhich an X-ray tube and a film are simultaneously revolved,cephalometric imaging in which an X-ray tube is greatly detached from afilm during imaging, and the like are performed. The X-ray panoramicimaging for dentistry is an imaging method in which curved crosssections are sequentially imaged along a tooth row, and the tomographicimages are spread out and displayed as one panoramic image showingconditions of the tooth row and tissue and bone around it.

In a conventional panoramic imaging apparatus, a rotative arm mountingan X-ray generating device and a two-dimensional X-ray detecting device,which are disposed so as to face each other, while interposing theobject therebetween, is supported by, e.g. a back-forth/left-rightmovable unit and a rotative unit, which is designed to move around theobject along a complicated orbit simulating the shape of a dental arctherebetween. An example of a tomograph for dentistry of this kind isdisclosed in Japanese Unexamined Patent Publication No.Hei06-78919.

Also, as a dental X-ray imaging apparatus, an X-ray CT apparatus whichcan obtain a horizontal tomographic image of a single tooth, other thana panoramic image, has been proposed. The apparatuses of this kindinclude, for example, a medical X-ray tomograph of the type mentioned inJapanese Unexamined Patent Publication No.Hei 09-122118 (the firstconventional technique) and a panoramic X-ray imaging apparatus of thetype mentioned in Japanese Unexamined Patent Publication No.Hei11-318886 (the second conventional technique). A known example of ageneral medical CT apparatus using a cone beam X-ray is mentioned inJapanese Unexamined Patent Publication No.Hei 10-192267 (the thirdconventional technique). According to this apparatus, a tomographicimage covering a wide region of the object can be obtained so as to beapplicable to imaging on the jaw, including the dental arc.

Meanwhile, a technique involved with an X-ray CT imaging method andapparatus for irradiating a cone beam X-ray only to one portion of theobject and obtaining an arbitrary tomographic image or athree-dimensional image of the portion is mentioned as an example of alocal irradiation X-ray CT apparatus in Japanese Unexamined PatentPublication No.2000-139902 (the fourth conventional technique).Specially, in a dental application, a cone beam X-ray is irradiated withrotation not to the whole jaw including the dental arc, but only to alocal region limited around a tooth and jaw joint, to thereby reduce theexposure dose, and a CT image and a three-dimensional image of highresolution are provided.

It is known that in any of the above-described conventional techniques,it takes a very long time (20 minutes to about one hour) for imagecalculation processing after imaging until an image is presented on animage display device.

The present invention has been developed in consideration of the abovedescribed points, and its object is to provide an X-ray CT apparatuswhich can greatly shorten the time taken for image processing when apanoramic image showing conditions of a tooth row, teethridge, andtissue and bone around them is obtained.

SUMMARY OF THE INVENTION

To achieve the above-stated object, an X-ray CT apparatus according tothe present invention includes X-ray generating means for generating anX-ray, X-ray detecting means for two-dimensionally detecting an X-raydose which is transmitted through an object to be examined, holdingmeans for holding the X-ray generating means and the X-ray detectingmeans so that the object is located therebetween, first rotation drivingmeans for driving the holding means to rotate the X-ray generating meansand the X-ray detecting means around the object, containing meansattached to the holding means for containing the first rotation drivingmeans, image processing means for producing an image involved with theobject on the basis of the X-ray dose detected by the X-ray detectingmeans, and image display means for displaying an image created by theimage processing means. The X-ray CT apparatus further includes secondrotation driving means for rotating, as one body, the holding means andthe containing means, wherein the second rotation driving meanscontained in the containing means is in parallel with a rotation centerof the first rotation driving means and in a different relation of therotation center position than that of the first rotation driving means,and drive control means for controlling the driving of the firstrotation driving means in a first imaging mode and separately drivingthe second rotation driving means and the second driving means in asecond imaging mode.

The first rotation driving means is designed to rotate the holding meansfor holding the X-ray generating means and the X-ray detecting means,which are arranged opposite to each other, with respect to the object.An X-ray is irradiated while the X-ray generating means and the X-raydetecting means rotate around a local region in the vicinity of therotation center of the first rotation driving means. By the secondrotation driving means, the holding means and the first rotation drivingmeans are driven and rotate as one body, whereby the rotation center ofthe first rotation driving means is revolved on a predetermined circle.Then in the first imaging mode (CT imaging), the location of a localX-ray irradiating region is determined by moving the rotation center ofthe first rotation driving means on the approximate circumference of adental arc using the second rotation driving means. In the secondimaging mode (panoramic imaging), the rotation center of the firstrotation driving means moves along the approximate circumference of theshape of the dental arc, while the rotation angle, i.e. the imagingdirection can be properly adjusted so that the irradiating direction issubstantially perpendicular to the dental arc.

Because a most suitable panoramic image can be obtained in this way byusing a simple mechanical means, complicated image calculatingprocessing becomes unnecessary, whereby the time necessary for imageprocessing in obtaining a panoramic image can be greatly shortened. Whenthe distance between the X-ray generation source and the object variesdue to the difference between the shape of the locus of movement andthat of the actual dental arc, so that the expansion ratio of thefluoroscopic image varies depending on the tooth position, a properpanoramic image can be obtained by correcting the variation in the imagecalculating processing on each of the acquired local data in synchronismwith the rotation angle of the rotation mechanism moving along theapproximate circumference of the dental arc and reconstructing the wholeimage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic side view showing the structure of an X-ray CTapparatus according to the present invention, and also showing a crosssectional structure of one portion.

FIG. 2 is a partial enlarged view of the cross sectional structure ofthe one portion shown in FIG. 1 for easy understanding.

FIG. 3 is a diagram showing a procedure used for positioning in a casewhere imaging is performed by the X-ray CT apparatus according to theembodiment of FIG. 1.

FIG. 4 is a partial enlarged view of FIG. 3.

FIG. 5 is a diagram showing an operation in a case where panoramicimaging is performed by the X-ray CT apparatus according to theembodiment of FIG. 1.

FIG. 6 is a partial enlarged view of FIG. 5.

FIG. 7 is a diagram showing differences between the rotation center of afirst rotation system 6 and centers of teeth in a case where therotation angle of the second rotation system is changed at everyirradiation.

FIGS. 8 a and 8 b are diagrams respectively showing the state before andafter correction of an expansion ratio of fluoroscopic images taken byeach cone beam X-ray in the arrangement of FIG. 7.

FIG. 9 is a top view of a variation of a rotation mechanism of thesecond rotation system in the X-ray CT apparatus according the presentinvention.

FIG. 10 is a diagram showing a rotation radius drawn by the X-ray CTapparatus of FIG. 9.

FIG. 11 is a diagrammatical sectional view of the structure of apositioning device used in the X-ray CT apparatus according to thisembodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, a preferable embodiment of an X-ray CT apparatus accordingto the present invention will be described with reference to theaccompanying drawings. FIG. 1 is a side view showing an example of thestructure of the X-ray CT apparatus according to the present invention,which also shows the structure of a cross section of a region in whichthe rotation system is disposed. FIG. 2 is a partial enlarged viewshowing the partial cross sectional structure of FIG. 1 for easyunderstanding.

This X-ray CT apparatus includes a fixing column 1, a rotative arm 2, anX-ray generating device 3, a two-dimensional X-ray detecting device 4, afirst rotation system 6, a second rotation system 5, a chair 8, and ahead holder 9. Fixing column 1, being in a reverse L-shape, is supportedby column portion 1 a and houses the second rotation system 5 and firstrotation system 6 at one end thereof. Rotative arm 2 is suspended fromthe end of the fixing column 1. First rotation system 6 holds therotative arm 2 in the suspended state and rotates the arm 2 at apredetermined speed around the rotation center of the rotation shaft 6 aat the end of fixing column 1. The second rotation system 5 is designedto rotate the whole first rotation system 6 at a predetermined speedaround rotation axis 5 a. That is, the positional relationship betweenthe second rotation system 5 and the first rotation system 6 is suchthat they are arranged in parallel and each have a different rotationalcenter, although the rotation systems are contained in a commoncontaining unit. The second rotation system 5 and the first rotationsystem 6 will be described later in more detail.

Although the second rotation system 5, the first rotation system 6, andthe rotative arm 2 attached thereto are arranged over the head of object7 in the above-described example, they also may be arranged in thedirection of the feet of object 7. By doing so, when object 7 sits onthe chair in accordance with the imaging position of the X-ray CTapparatus, the object 7 need not worry about collision with the rotationsystem, including the rotative arm 2.

The X-ray generating device 3, which is designed to generate an X-ray,is located on one end of rotative arm 2. The X-ray generating device 3includes a collimator device 3 c for narrowing down the X-rays 3 birradiated from the X-ray generation source 3 a inside the X-raygenerating device 3 into a cone-shaped beam. The two-dimensional X-raydetecting device 4, which is arranged opposite to X-ray generatingdevice 3, is designed to two-dimensionally detect an X-ray dosetransmitted through the object, and it is installed in the other end ofthe rotative arm 2. That is, the X-ray generating device 3 and thetwo-dimensional X-ray detecting device 4 are arranged opposite to eachother on respective ends of the rotative arm 2. Rotative arm 2 is drivenby the first rotation system 6 to rotate it by approximately 405° arounda rotation center near the tip of the fixing column 1. Although theimaging range is 360° , the rotation range is wider than it this 45°,because imaging is started when the rotation speed becomes constant. Therotation range is widely set for accelerating the rotation until therotation of the first rotation system 6 becomes constant anddecelerating the rotation until the first rotation system 6 stops afterimaging, and so it is not limited to 45°. After imaging is started, theX-ray generating device 3 irradiates an X-ray in pulse form insynchronism with image acquisition, and X-ray exposure to the object isthus reduced. The timing thereof is controlled by a positional detectionencoder built into the first rotation system 6. Inside column portion 1a of the fixing column 1, a control system for control of the imagingapparatus is installed.

Image data acquired by the two-dimensional X-ray detecting device 4 issent to an image processing device 12. The image processing device 12 isinstalled in an operation room distant from the imaging room in whichthe X-ray CT apparatus is installed. Image processing device 12 performscalculation processing on the received image data, reconstructs atwo-dimensional tomographic image, a CT image, or a three-dimensionalimage, and presents the image on the image display device 13. Imagedisplay device 13 includes an input device, such as a keyboard and amouse (not shown), and it operates so that the image processing device12 functions as a computer device. Because conditions of imagereconstruction can be input from this input device, it is possible toinput which of the two-dimensional tomographic image, CT image, thethree-dimensional image, and the panoramic image is to be a subject ofthe reconstruction.

Rotative arm 2 is supported rotatably and horizontally relative to thefixing column 1. In this embodiment, the rotative arm 2 is constructedto be an up-down dual structure, including second rotation system 5 andfirst rotation system 6. Second and first rotation systems include arotation supporting mechanism using a bearing, a driving mechanism and aposition detecting mechanism rotated by the combination of a servomotorand a gear, and a cable processing mechanism of the rotation unit. Therotation center 5 a of the second rotation system 5 is fixed relative tothe fixing column 1 and the rotation center 6 a of the first rotationsystem 6 is fixed relative to the rotative arm 2. The rotation centers 5a and 6 a are spaced from each other by a fixed distance d. Rotationcenter 6 a of the first rotation system 6 is rotated by a driving device5 b of the second rotation system 5 around a rotation axis on therotation center 5 a of the second rotation system 5. And, driving device5 b (including the position detecting device) of the second rotationsystem 5 and a cable processing mechanism (concrete structure being notshown) are contained inside the fixing column 1. The driving device 6 bof the first rotation system (including the position detecting device)and the cable processing mechanism 6 c are contained in the upper partof the second rotation system 5. The distance d between the two rotationcenters 5 a and 6 a substantially corresponds to the size of the dentalarc of object 7, e.g. a diameter of around 70 to 100 mm. The firstrotation system 6 has to be rotated by 360° or more (around 405°) toacquire CT image data. On the other hand, because the second rotationsystem 5 is provided for the purpose of making a rotation similar to thedental arc, it is sufficient for it to rotate ±120° at the maximum.

Further, because there are individual differences in remaining teeth andthe like of the object 7, the maximum rotation angle of the secondrotation system 5 is not limited to ±120°, and it can be arbitrarily setusing an input device.

The cable processing mechanism of the second rotation system 5 and thefirst rotation system 6 is commonly provided by using a guiding railalong the movement of the cable caused by the rotations. Accordingly, aplurality of cable processing mechanisms are unnecessary, and so theinstallation space of the mechanism can be miniaturized.

Meanwhile, object 7 sits on the chair 8, which can be moved upward anddownward. The position of examining region 7 a of the object 7 isdetermined relative to the height of the imaging center of the imagingapparatus. The angle of the chair back 8 b of the chair 8 can beadjusted at an arbitrary angle. The position of object 7 in a back-frontdirection is substantially adjusted by combining the angle adjustment ofthe chair back 8 b and the position adjustment effected by up-downmovement provided by the mechanism 8 a. A head holder 9 is provided inthe rear of the chair back 8 b and is adjustable upward and downward,backward and forward, and left and right in accordance with the seatedheight of the object 7 and the position of the examining region, so asto fix the head of the object 7 to a desirable position after adjustmentof the chair 8. The head of the object 7 is moved to a desirableposition by an operator and fixed by head band 9 b or the like. Thecenter of the examining region 7 a of the object 7 can be adjusted tothe rotation center 6 a of the rotative arm 2 (rotation center of firstrotation system 6 mentioned above) by the operator.

Chair 8 is moved upward and downward by the up-down movement mechanism 8a. If the head holder 9 is attached to the angle-adjustable chair back 8b, it is not necessary to use a dedicated chair according to theembodiment. That is, if the specifications of the up-down stroke and thelike are fulfilled, a chair of the type used, e.g. by a hairdresser, maybe utilized. Further, a therapeutic chair of the type used inotorhinolaryngology also may be utilized.

Further, as seen in FIG. 1, a state in which the back of object 7 isdirected to column 1 a, which is perpendicular to the floor on whichfixing column 1 is installed, is shown. However, because the imagingapparatus is separated from the chair in this embodiment, the angle ofthe object's position 20 relative to column 1 a is not limited as longas a rotation range resides in CT imaging. For example, imaging also canbe performed while the object 7 is placed so as to face the column 1 a.In this case, by setting a projector of an optical marker forpositioning on the column, the optical marker can be projected from adirection opposite to object 7, whereby positioning becomes easy.Alternatively, the setting direction of column 1 a and chair 8 can befreely set in relation to the layout of the imaging room where theapparatus is installed.

FIG. 3 is a diagram showing the procedure for positioning the objectwhen imaging is performed by the X-ray CT apparatus according to thisembodiment, and FIG. 4 is a partial enlarged view of FIG. 3. When CTimaging is performed locally on one or two teeth only, the secondrotation system 5 is revolved so as to match center 7 b of the regionwhere the tooth 11 a to be imaged is located, using the rotation center6 a of first rotation system 6 (rotative arm) of the X-ray CT apparatus.However, circle 10, the rotation radius of which corresponds to theconstant distance d between the rotation center 5 a of the secondrotation system 5 and the rotation center 6 a of the first rotationsystem 6, is made substantially to coincide with the shape and size ofdental arc 11. Accordingly, circle 10 does not always coincide withdental arc 11 depending on the position of the tooth being examined.That is, the center 7 b of the examining region cannot be matched withthe center 6 a of the rotative arm only by making object 7 sit on thecenter in the left-right direction of the chair 8.

If imaging region 7 a has a sufficient size to include several teeth, sothat the examining region is located in the vicinity of the center ofthe imaging range 7 a, imaging is sufficiently performed even when thecenter 7 b of the examining region is spaced a little from the center 6a of the rotative arm. However, when the imaging region is limited toone or two teeth because of a limitation of the detectable size of thetwo-dimensional X-ray detecting device 4, or when the shape and size ofthe dental arc 11 is individually different as between an adult and achild, so that apparent differences occur between the shape and size ofthe circle 10, whose rotation radius corresponds to the distance dbetween rotation centers 5 a and 6 b of the second and rotation system 5and the first rotation system 6 in the X-ray CT apparatus according tothis embodiment, and those of the object's dental arc, resulting in thetooth 11 a possibly being out of the imaging region 7 a, it is necessaryto match the center 7 b of the examining region and the rotation center6 a of the rotative arm as accurately as possible.

According to this embodiment, the object's position in the back-frontdirection is adjusted by properly combining the angle of adjustment ofthe chair back 8 b of the chair 8 and the up-down position adjustment bythe up-down movement mechanism 8 a to substantially match center 7 b ofthe examining region 11 a with the rotation center 6 a of the firstrotation system 6 (rotative arm 2). After fixing the head of the objectto the head holder 9 a of the chair 8, the head position is fineadjusted by using the back-front and left-right movement mechanism ofthe head holder 9. In this manner, the center 7 b of the examiningregion 11 a can be completely matched with the center 6 a of the firstrotation system 6 (rotative arm 2).

In the above-described positioning procedure, the positioning isgenerally performed with reference to a linear optical marker projectedto the body surface of the object 7 in the state in which the object'smouth cavity is closed. Therefore, in some cases, it is difficult tocheck from outside whether or not the position of the tooth imagedcompletely coincides with the center of the imaging range.

In this case, a more accurate method of positioning is applicable, inwhich positioning is performed using an optical marker from outside, asdescribed above, the direction of the rotative arm 2 of the imagingapparatus is then changed, and X-ray fluoroscopic imaging is performedfrom two orthogonal directions, whereby the position of the object 7 isremotely and finely adjusted, while the position of the teeth isvisually checked on a fluoroscopic image. In this case, the positionadjustment can be carried out more precisely and accurately by directlyfinely adjusting the position of the head of object 7 by remotelycontrolling the head holder 9, rather than by performing fine adjustmentwhile moving the object 7. Further, from the viewpoint of safety, too,the distance of movement of the object 7 is desirably kept to a minimumin order to prevent the object 7 from touching the X-ray CT apparatus,and to reduce any external force applied to the object 7 while movingthe object 7. In the case of the X-ray CT apparatus according to thisembodiment, it is characteristic that, after adjusting the roughposition of the object in the apparatus, the head holder of the chair 8is separately finely adjusted. According to this embodiment, the rangeof fine adjustment to the head is around ±15 mm at most. Accordingly,the burden on the object caused by movement of the head holder 9 is muchsmaller in comparison with the case in which the position adjustment iscarried out by moving the entire object (±50 mm to the maximum).Furthermore, it is possible to improve the accuracy of positioning andto shorten the time taken therefor.

Further, other than positioning using the rotation mechanism, it is alsoapplicable to use the second rotation system only in panoramic imaging,and positioning of the object 7 is performed by combining up-downmovement of the chair and up-down, back-forth, and left-right movementof the head holder 9.

Since the rotative arm 2 rotates around the head holder 9 to irradiate acone beam X-ray, head holder 9 a to which the cone beam X-ray 3 b isirradiated is desirably made of a material which is permeable toradioactive rays and has enough strength to hold and fix the head, suchas carbon fiber, so that the head holder 9 a does not become an obstacleto image data acquisition by reason of the fact that X-rays are absorbedthereby.

An advantage of using the head holder 9 is that, since the back of thehead of the object 7 is fixed by the head holder 9 a, safety in a regionwhere the object 7 cannot visually check (i.e. at the back of the head)by himself/herself is ensured, if it is ensured in the apparatus thatthe head holder 9 a does not touch the main body of the X-ray CTapparatus during rotation of the rotative arm 2.

Although only the head holder 9 is used as means for fixing andpositioning the object 7 in this embodiment, the fixing and positioningmeans is not limited to adjustment of the head holder 9. It is possibleto utilize a combination of a chin rest and an ear rod, or a fixingdevice using a dental articulation model produced in accordance with adenture model of each object, in combination with the head holder. Ifthose devices are constructed so that it can be fine adjusted back andforth, and left and right, a similar positioning function can berealized by fine adjusting their position.

In the state in which the position of the object 7 is fixed, asdescribed above, CT imaging is performed by revolving the rotative arm2, while cone beam X-ray 3 b is irradiated from the X-ray generatingdevice 3. In accordance with the rotation angle of rotative arm 2, thetwo-dimensional X-ray detecting device 4 mounted opposite to the X-raygenerating device 3 (not shown) at the other end of rotative arm 2 isrotated from the position of two-dimensional X-ray detecting device 4 tothat of two-dimensional X-ray detecting device 41. Eventually,fluoroscopic image data over 360° of diagnostic region 11 a is acquired.The acquired image data is subjected to calculation processing at imageprocessing device 12, a two-dimensional tomographic image or athree-dimensional image is reconstructed, and the image is displayed onthe image display device 13.

-   -   The execution of the above-described imaging process can be        outlined in the following order of steps (1) to (5):(1) An        imaging region of object 7 is substantially positioned by        rotating the second rotation system 5.    -   (2) The position of object 7 is fixed by finely adjusting the        head holder 9 of chair 8.    -   (3) If necessary, X-ray fluoroscopic imaging is performed from        two orthogonal directions, and the object's position is finely        adjusted while the position of the teeth is visually checked on        the fluoroscopic image.    -   (4) CT image data is acquired by rotating the first rotation        system 6 (rotative arm 2) while the cone beam X-ray 3 b is        irradiated.    -   (5) The acquired image data is subjected to calculation        processing in image processing device 12 to reconstruct a        two-dimensional tomographic image or a three-dimensional image,        and the image is displayed on image display device 13.

FIG. 5 is a diagram showing the operation when panoramic imaging isperformed by the X-ray CT apparatus according to this embodiment, andFIG. 6 is a partial enlarged view thereof. First, the center in theleft-right direction of the object 7 is paced in agreement with therotation center 5 a of the second rotation system 5. If the center inthe left-right direction of chair 8 is preset to be just below therotation center 5 a of second rotation system 5, position adjustment tothe object 7 in the left-right direction is scarcely necessary, exceptfor fine adjustment of the head holder 9. However, it is here postulatedthat the dental arc 11 of the object 7 is symmetrical with respect tothe center of the object 7 in the left-right direction. Subsequently,the position of the object 7 in the back-front direction is properlyadjusted by combining the angle adjustment to chair back 8 b of chair 8and the position adjustment to up-down movement mechanism 8 a in theup-down direction, so as to substantially match the shape and size ofdental arc 11 with circle 10, the rotation radius of which correspondsto the distance d between rotation center 5 a of the second rotationsystem 5 and the rotation center 6 a of the first rotation system 6.After fixing the rear of the head to the head holder 9, the positionthereof is finely adjusted by a back-front and left-right movementmechanism of head holder 9. In this manner, the rotation center 6 a ofthe first rotation system 6 is located on circle 10.

As described above, in the state in which the position of the object 7is fixed, rotative arm 2 is revolved by first rotation system 6 inaccordance with the rotation angle of the second rotation system 5 whilethe second rotation system 5 is revolved. A tomogram of the dental arc11 is obtained by irradiating the cone beam X-ray 3 b in a directionperpendicular to each tooth of the dental arc 11 which does notinterfere with an opposite tooth of the dental arc 11. Because of thedifferences in the shape and size between the actual dental arc 11 andthe circle 10, which has a rotation radius corresponding to the distanced between rotation center 5 a of the second rotation system 5 and therotation center 6 a of first rotation system 6, the distance betweenX-ray generation source 3 a and the object 7 varies depending on theposition of the tooth being examined. As a result, the expansion ratioand the density of the fluoroscopic image projected to thetwo-dimensional X-ray detector 4 will vary. That is, when a tooth 1 lais imaged, although the cone beam X-ray 3 b 1 irradiated from the X-raygenerator 3 is transmitted through the center of rotation center 6 a 1of the first rotation system 6 and the tooth 11 a, there is a littledistance between the rotation center 6 a 1 of the first rotation system6 and the center of the tooth 11 b.

In a similar manner, when tooth 11 b is imaged, although cone beam X-ray3 b 2, which is irradiated from the X-ray generator 3 is transmittedthrough rotation center 6 a 2 of first rotation system 6 and the centerof tooth 11 b, there is a little distance between rotation center 6 a 2of first rotation system 6 and the center of tooth 11 b. When tooth 11 cis imaged, although cone beam X-ray 3 b 3, which is irradiated the X-raygenerator 3, is transmitted through rotation center 6 a 3 of firstrotation system 6 and the center of tooth 11 c, there is a littledistance between rotation center 6 a 3 of first rotation system 6 andthe center of tooth 11 c.

Accordingly, after the differences therebetween are corrected in theimage calculating processing in synchronism with the rotation angle ofsecond rotation system 5, partial image data at each angle are joined toreconstruct a continuous image over the whole angle. Thus, an accuratepanoramic image can be obtained.

FIG. 7 is a diagram showing the differences between the rotation centerof first rotation system 6 and the center of the respective teeth in thecase where the rotation angle of the second rotation system 5 is variedeach time. As the imaging of a tooth is gradually shifted from the leftend to the right end of the dental arc, the rotation center 6 a of firstrotation system 6 moves on circle 10. Accordingly, the irradiation angleof the cone beam X-rays 3 b 1 to 3 b 6 is gradually as depicted byvaried arrow 70. By irradiating cone beam X-rays 3 b 1 to 3 b 6,fluoroscopic images b1 to b6 are obtained by the two-dimensional X-raydetecting device 42. Because of the differences between the shape andsize of the actual dental arc and those of circle 10, which has arotation radius corresponding to the distance d between rotation center5 a of the second rotation system 5 and the rotation center 6 a of thefirst rotation system 6, a difference occurs in the distance betweenX-ray generator 3 a and the object 7 depending on the position of thetooth being examined. Resultingly, a difference occurs in the expansionratio and density of the fluoroscopic image projected on two-dimensionalX-ray detecting device 4.

FIGS. 8 a and 8 b are diagrams showing the state of fluoroscopic imagesb1 to b6 obtained by cone beam X-rays 3 b 1 to 3 b 6 before and aftercorrection of expansion ratio, respectively. As shown in FIG. 8 a, eachof the fluoroscopic images b1 to b5 before the correction of theexpansion ratio substantially has the same size. By multiplying thosefluoroscopic images b1 to b6 before correction by expansion ratios k1 tok6, respectively, in accordance with the difference between the rotationcenter of first rotation center 6 (point on circle 10) and the center ofeach tooth, the sizes of the fluoroscopic images b1 to b6 can becorrected, as shown in FIG. 8 b. Then, a panoramic image isreconstructed on the basis of the size-corrected image. Meanwhile,although the density of the image is not shown in the figure, it isneedless to say that the density is also corrected.

In this correction processing, the amount of information to be dealtwith is small, and so the correction processing itself is simple.

Further, if two kinds of dental arcs of standard size are preparedrespectively for adults and children as the shape and size of dental arc11 to serve as a reference in the correction, the correction can beautomatically done with reference thereto and only two kinds of tablesof correction coefficient used with the software are necessary.Accordingly, it is possible to reduce the memory capacity installed inthe image processing device 12 and shorten the time taken for imageprocessing. If the shape and size of the dental arc 11 customized foreach individual can be produced other than those of the reference sizefor the software, it is needless to say that more accurate correctioncan be done.

The execution of the above panoramic imaging procedure is outlined inthe following order of steps (1) to (6):

-   -   (1) By combining the angle adjustment of the chair back 8 b of        chair 8 and the up-down position adjustment of the up-down        movement mechanism 8 a, the imaging region of object 7 is        positioned so that the trajectory 10 (circular trajectory) drawn        by rotation center 6 a of rotative arm 2 (first rotation system        6) substantially coincides with the dental arc 11.    -   (2) The head holder 9 of chair 8 is fine adjusted to fix the        position of object 7.    -   (3) The second rotation system 5 is revolved to adjust one end        of dental arc 11 (back tooth) to an irradiation starting        position. At the same time, the rotation angle of the rotative        arm 2 is adjusted in a direction perpendicular to dental arc 11        which does not interfere with an opposite tooth of the dental        arc 11, and irradiation of cone beam X-ray 3 b is started.    -   (4) While the second rotation system 5 is revolved along dental        arc 11, the rotation angle of the rotative arm 2 is adjusted in        a direction perpendicular to the dental arc 11, which does not        interfere with each of the opposite teeth of dental arc 11, cone        beam X-ray 3 b is sequentially irradiated, and thus partial        fluoroscopic image data is acquired at each rotation angle of        second rotation system 5.    -   (5) The imaging procedure is completed when the position of the        data acquisition of a partial fluoroscopic image reaches the        other end of the dental arc (back tooth opposite to the starting        position).    -   (6) After the expansion ratio and density of the partial        fluoroscopic image data acquired at each rotation angle of the        second rotation system 5 are corrected in the image processing        device 12 in synchronism with each rotation angle of the second        rotation system 5, a continuous panoramic image of the dental        arc 11 over the whole angle is reconstructed and presented on        image display device 13.

FIG. 9 is a diagram showing a variation of the rotation mechanism of thesecond rotation system of the X-ray CT apparatus according to thepresent invention, and it is provided as a top view of the X-ray CTapparatus of FIG. 1. FIG. 10 is a diagram showing a rotation radiusdrawn by this X-ray CT apparatus, and it represents an enlarged viewcorresponding to FIG. 4. Although the distance (rotation radius) dbetween the rotation center 5 a of the second rotation system 5 and therotation center 6 a of the first rotation system 6 is fixed in the X-rayCT apparatus shown in FIG. 1, in the X-ray CT apparatus shown in FIG. 9,the rotation radius d can be freely changed, and the rotation center 6 aof first rotation system 6 moves on a complicated trajectory alongdental arc 11. A linear driving system includes driving means 14 a, suchas a servomotor, mounted on the second rotation system 5, and a lineardriving mechanism 14 b. such as a feed screw and a rack and pinion,driven by driving means 14 a. In the linear driving system, the rotationcenter 6 a of the first rotation system 6 is moved in the direction ofarrow 14 c, and thus the distance (rotation radius) d between therotation center 5 a of the second rotation system 5 and the rotationsystem 6 a of the first rotation system 6 is moved to a desiredposition. In this manner, by use of the linear driving system forcorrecting the position of the rotation center 6 a of the first rotationsystem 6, the trajectory of the rotation center 6 a of the firstrotation system 6 is drawn on a curved line 10 a taken along dental arc11, as shown in FIG. 10. Accordingly, because a difference between therotation center 6 a of first rotation system 6 and the center of teeth11 does not occur, correction processing in the image calculatingprocessing becomes unnecessary, and so the calculation time can beshortened.

The moving range of the rotation center 6 a of the first rotation system6, which is moved by the linear driving system, is a differencecorresponding to the distance between the center of each tooth of thedental arc 11 and the rotation radius 10 in FIG. 4, around ±15 mm beingenough. Further, if the moving distance is smaller, a loadsupporting/driving device can be miniaturized, and bending and the likeoccurring due to the weight of apparatus can be reduced. Therefore,position adjustment can be accurately carried out using a simplemechanism.

According to the X-ray CT apparatus shown in FIG. 9, accuratepositioning is easily performed in the apparatus, regardless of theshape and size of the dental arc of the object. If the apparatus isconstructed so that this adjusting mechanism of the rotation radius canbe remote controlled, fine adjustment of the head holder 9 becomesunnecessary, whereby it is possible to greatly reduce the burden on theobject 7 and to simplify the mechanism for adjusting the position ofchair 8 and the fixing object 7. Further, image correction in regard todifferences in the expansion ratio and the density of a fluoroscopicimage obtained in the panoramic imaging also becomes unnecessary,whereby the time taken for the image calculation processing can beshortened.

Since it is not necessary to move the object with use of this X-ray CTapparatus, the following imaging method can be conducted. That is, asshown in FIG. 10, X-ray CT imaging is sequentially and repeatedlyexecuted plural times (nine times in FIG. 10) on local regions 7 a to 7i, each including two to three teeth, so as to cover the whole dentalarc 11, and thus CT image data combining a plurality of local imagingregions 7 a are acquired. In this manner, even when an X-ray detectingdevice having a small FOV is used, CT image data for presenting thewhole dental arc can be acquired.

The execution of the above imaging procedure on local region can beoutlined in the following order of steps (1) to (7):

-   -   (1) An imaging region of object 7 is positioned and fixed so        that a trajectory drawn by rotation center 6 a of the rotative        arm 2 (dental arc-shaped trajectory) coincides with the dental        arc 11 of object 7.    -   (2) The rotation center 6 a of the rotative arm 2 is adjusted to        the center of a back tooth located at one end of dental arc 11,        i.e. local region 7 a.    -   (3) A cone beam X-ray 3 b is irradiated while the rotative arm 2        is revolved, and thus CT image data is acquired.    -   (4) The second rotation system 5 is revolved to adjust the        rotation center 6 a of the rotative arm 2 to the center of local        region 7 b adjacent to and partially overlapping with local        region 7 a, the CT image data of which was acquired above.    -   (5) CT image acquisition and positioning are repeatedly carried        out on local regions 7 b to 7 i along dental arc 11.    -   (6) The imaging procedure is completed when CT image data        acquisition on the other end of the dental arc 11 (center of        back tooth on the end opposite to the starting position, i.e.        local region 7 i) is finished.    -   (7) The CT image data acquired is subjected to calculation        processing in the image processing device 12, and an image of        the whole dental arc 11 is reconstructed and displayed on the        image display device 13.

According to this method of imaging local regions, an image of higherresolution can be obtained on an identical region. Further, it is alsopossible to extract fluoroscopic image data along the dental arc in adirection perpendicular to the tooth row from the above-mentioned dataand to reconstruct a panoramic image. Similarly, a tomographic image anda three-dimensional image of an arbitrary cross section can bereconstructed. Further, by limiting the X-ray irradiating region to alocal region, the exposure dose of object 7 can be reduced. According tothis embodiment, the exposure dose can be reduced as the number of timesof CT imaging becomes smaller.

When the method of imaging a local region is applied to the imagingmethod of FIG. 3 to FIG. 6, it is needless to say that similar imagingcan be performed by sequentially repeating the local CT imaging, whilepositioning the patient at every CT imaging on the local region.

Meanwhile, although the X-ray CT apparatus according to this embodimentis suitable for dentistry, it is needless to say that this technique isnot limited to dentistry and is applicable to a general X-ray CTapparatus. For example, the method is applicable when an object to beexamined is larger than the imaging range of the X-ray CT apparatus, aswell as in the case where sequential local CT imaging is performed onthe whole object, or in the case where panoramic imaging is performedfrom the inside of a cylindrical body simulating the shape of a dentalarc.

FIG. 11 is a diagram showing the structure of a positioning device usedin the X-ray CT apparatus according to this embodiment. The positioningdevice 20 includes a dental articulation unit 15 produced in accordancewith a dental model of each object to be examined and a flange 16 joinedto the dental articulation unit 15 via a joint unit 15 a. Flange 16 ismade of a thin plate fixed onto a surface parallel to the dentalarticulation unit 15. When the dental articulation unit 15 is placed inthe object's mouth, flange 16 is exposed outside the mouth of the object7 via joint unit 15 a. On flange 16, line marks 16 a to 16 c are incusedalong orthogonal axes 17 and 18. Further, on both sides of therespective line marks 16 a to 16 c, scale marks 16 d are incused atregular intervals around line mark 16 a to 16 c. Scale mark 16 d givesan indication of the distance when the object's position is shifted inCT imaging so as to match the center of the region imaged with rotationcenter 6 a of the rotative arm 2.

The X-ray CT apparatus is provided with a projector (not shown) forprojecting an optical marker to the face of object 7 from threedirections corresponding to the line marks 16 a to 16 c of the flange16, which is positioned so that an intersecting point of those lightaxes passes through rotation center 5 a of the second rotation system 5.Then, by fine adjusting the position of the object 7 wearing thepositioning device 20 so that line marks 16 a to 16 c coincide with theoptical marker, the position of the rotation center 5 a of the secondrotation system 5 can be visually checked from the outside of the mouth.Accordingly, accurate positioning can be realized only with positioningbased on the optical marker, without performing X-ray fluoroscopicimaging.

By changing the position of attachment of the joint unit 15 a and flange16, the positional relation between the dental articulation unit 15 andthe rotation center 5 a of the second rotation system 5 can be changed.That is, an adjustment can be performed so that the trajectory (circulartrajectory) 10 drawn by rotation center 6 a of the rotative arm 2substantially coincides with the dental arc 11. In this adjustment, itis useful to prepare in advance a full-scale figure like the one shownin FIG. 11 for checking the state of correspondence of the circulartrajectory 10 and the dental arc 11 and to combine them while checkingthat the figure corresponds with the real thing. Further, this figurealso may be used when it is checked that the intersecting point of thelight axes of the optical marker projector from three directions passesthrough the rotation center 5 a of the second rotation system 5.Meanwhile, although the above description relates to a case in whichpositioning device 20 is applied to the X-ray CT apparatus shown in FIG.3 to FIG. 6, it also may be applied similarly to the X-ray CT apparatusshown in FIGS. 9 and 10 by using a dental arc-shaped trajectory 10 a,instead of the circular trajectory 10. Further, the above-describedpositioning processing may be automatically carried out by detectingpositioning device 20 using a camera or the like and performing imageprocessing.

The execution of the imaging procedure using the above-describedpositioning device 20 is outlined in the following order of steps (1) to(4):

-   -   (1) Positioning device 20 is worn by object 7, in which dental        articulation unit 15 is produced on the basis of a dental model        of the object 7.    -   (2) The position of attachment of the joint unit 15 a and flange        16 is adjusted to connect and fix them so that the trajectory        (circular trajectory) 10 drawn by the rotation center 6 a of the        rotative arm 2 or the dental arc-shaped trajectory 10 a        substantially coincides with the dental arc 11.    -   (3) Dental articulation unit 15 of the device joined and fixed        is put on to dental arc 11.    -   (4) Line marks 16 a to 16 c of the flange 16 are made        corresponding with the optical marker of the apparatus and the        object is positioned.

According to the above-described embodiment, in the cone beam X-ray CTimaging apparatus for dentistry, positioning in local CT imaging can beeasily carried out with a simple rotation mechanism, and the positioningmechanism can be simplified. Further, panoramic imaging also can beperformed easily, and it is possible to greatly shorten the time forimage calculation processing and to simplify the image processingdevice. Further, it is possible to perform local CT imaging andpanoramic imaging without moving the object and to simplify the positionadjustment of the chair and the object fixing mechanism. Further, imagecorrection in regard to differences in the expansion ratio and densityof a fluoroscopic image of an imaging region becomes unnecessary. In anX-ray CT apparatus with a small FOV, too, CT image data of highresolution over the whole dental arc can be acquired by sequentiallyrepeating the local CT imaging plural times. Further, a panoramic imageof the whole dental arc, a tomographic image of an arbitrary crosssection, and a three-dimensional image can be reconstructed from thoseimage data.

Meanwhile, according to the above-described embodiment, rotative arm 2is constructed as a single unit. However, it is also available toconstruct the rotative arm as a dual unit and relatively slide each unitso as to freely extend and contract its length in its radius directionand adjust the distance between the X-ray generating device 3 and thetwo-dimensional X-ray detecting device 4.

INDUSTRIAL APPLICABILITY

As described above, in an X-ray CT apparatus according to the presentinvention, when a cone beam X-ray is irradiated to the whole jaw,including a dental arc, and to a local region, such as a tooth andaround the jaw joint, and a panoramic image showing conditions of thedental row, teethridge, and the tissue and bone around them is obtained,the time taken for image processings can be extremely shortened.

1. An X-ray CT apparatus comprising: X-ray generating means forgenerating an X-ray; X-ray detecting means arranged opposite to theX-ray generating means for two-dimensionally detecting an X-ray dosewhich is transmitted through an object to be examined; holding means forholding the X-ray generating means and the X-ray detecting means so thatthe object is positioned therebetween; first rotation driving means fordriving the holding means to rotate on a locus of movement around theobject; support means for supporting the first rotation driving means;image processing means for producing an image of the object on the basisof the X-ray dose detected by the X-ray detecting means; and imagedisplay means for displaying the image produced in the image processingmeans, further comprising; second rotation driving means to integrallyrotate the holding means supported by the support means and the supportmeans in a manner such that the rotation center of the second rotationdriving means is parallel with the rotation center of the first rotationdriving means and is located at a different position from that of thefirst rotation driving means; and drive control means for controllingthe first rotation driving means in a first imaging mode and separatelycontrolling the first rotation driving means and the second rotationdriving means in a second imaging mode.
 2. An X-ray CT apparatusaccording to claim 1, wherein the drive control means performs controlfor execution of each of the first imaging mode and the second imagingmode.
 3. An X-ray CT apparatus according to claim 1, wherein the imageprocessing means reconstructs a two-dimensional tomographic image or athree-dimensional image of the object in the first imaging mode, andreconstructs a panoramic image of the object in the second imaging mode.4. An X-ray CT apparatus according to claim 1, wherein each of therotation center of the first rotation driving means and the secondrotation driving means is arranged so that the distance therebetween isdetermined on the basis of the size of an imaging region of the object.5. An X-ray CT apparatus according to claim 1, wherein the rotationangle of the second rotation driving means is determined so that each ofthe holding means and the first rotation driving means is located in apredetermined imaging region of the object.
 6. An X-ray CT apparatusaccording to claim 1, wherein, in the case of executing the secondimaging mode, differences in expansion ratio of a fluoroscopic image ofthe object, which occur due to differences in the distance between thelocus of movement of the center of the first rotation driving means andeach imaging region of the object, are corrected by an image calculatingprocessing.
 7. An X-ray CT apparatus according to claim 1, wherein saidholding means in a rotative arm; and the second rotation driving meansis rotatable around a center of rotation of the rotative arm on a locusof movement simulating the shape of an imaging region of the object,positions the local X-ray irradiating region in the first imaging mode,and adjusts the imaging direction in the combination of the position ofthe irradiating region and a rotation angle of the rotative arm in thesecond imaging mode.
 8. An X-ray CT apparatus according to claim 1,wherein said holding means is a rotative arm; and the second rotationdriving means is rotatable around a center of rotation of the rotativearm on a circumference simulating the shape of an imaging region of theobject, and has a mechanism for varying the diameter of the locus ofmovement of the center of the first rotation driving means.
 9. An X-rayCT apparatus according to claim 1, wherein by locally repeating thefirst imaging mode plural times along the imaging region of the objectwith the second rotation driving means, image data in the first imagingmode over the imaging region of the object are acquired, and a panoramicimage over the imaging region, a tomographic image or athree-dimensional image of an arbitrary cross section of the object isreconstructed from the image data.